1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a first print time required to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate an increased amount of heat before a plurality of recording media is conveyed through the fixing device continuously at an increased speed.
To address these requests, the fixing device may employ an endless belt having a decreased thermal capacity and therefore heated quickly by a heater. FIG. 1 illustrates a fixing device 20R1 incorporating an endless belt 100 heated by a heater 300. As shown in FIG. 1, a pressing roller 400 is pressed against a tubular metal thermal conductor 200 disposed inside a loop formed by the endless belt 100 to form a fixing nip N between the pressing roller 400 and the endless belt 100. The heater 300 disposed inside the metal thermal conductor 200 heats the entire endless belt 100 via the metal thermal conductor 200. As the pressing roller 400 rotating clockwise and the endless belt 100 rotating counterclockwise in FIG. 1 convey a recording medium P bearing a toner image T through the fixing nip N in a recording medium conveyance direction A1, the endless belt 100 and the pressing roller 400 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P.
Since the metal thermal conductor 200 heats the endless belt 100 entirely, the endless belt 100 is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating the increased amount of heat for high speed printing. However, in order to shorten the first print time further and save more energy, the fixing device is requested to heat the endless belt more efficiently. To address this request, a configuration to heat the endless belt directly, not via the metal thermal conductor, is proposed as shown in FIG. 2.
FIG. 2 illustrates a fixing device 20R2 in which the heater 300 heats the endless belt 100 directly. Instead of the metal thermal conductor 200 depicted in FIG. 1, a nip formation member 500 is disposed inside the loop formed by the endless belt 100 and presses against the pressing roller 400 via the endless belt 100 to form the fixing nip N between the endless belt 100 and the pressing roller 400. Since the nip formation member 500 does not encircle the heater 300 unlike the metal thermal conductor 200 depicted in FIG. 1, the heater 300 heats the endless belt 100 directly, thus improving heating efficiency for heating the endless belt 100.
FIG. 3 illustrates another fixing device 20R3 in which the heater 300 heats the endless belt 100 directly. Instead of the nip formation member 500 depicted in FIG. 2, the fixing device 20R3 includes a nip formation assembly 503 constructed of a base pad 501 and a low-friction sheet 502 wrapped around the base pad 501. As the endless belt 100 rotates counterclockwise in FIG. 3, it slides over the low-friction sheet 502 with a decreased friction therebetween, thus decreasing wear of the endless belt 100.
With the configurations of the fixing devices 20R1 and 20R2 described above, as the endless belt 100 rotates in accordance with rotation of the pressing roller 400, an upstream portion of the endless belt 100 disposed upstream from the fixing nip N in the rotation direction of the fixing belt 100 is pulled toward the fixing nip N by the rotating pressing roller 400. For example, an upstream portion 100a of the endless belt 100 of the fixing device 20R2 shown in FIG. 2, as it is pulled toward the fixing nip N, may strike an upstream edge 500a of the nip formation member 500 and therefore may be damaged or broken. Similarly, the low-friction sheet 502 of the fixing device 20R3 shown in FIG. 3 may strike an upstream edge 501a of the base pad 501 and therefore may wear. As the upstream portion 100a of the endless belt 100 strikes the upstream edge 501a of the base pad 501 no longer protected by the worn low-friction sheet 502, the upstream portion 100a of the endless belt 100 may be damaged or broken.
As the thinner endless belt 100 having a decreased mechanical strength is employed to shorten the first print time further and save more energy, a technology to minimize damage and breakage of the endless belt 100 is requested.